The present invention generally relates to pressure vessels, such as water heater storage tanks and, in a preferred embodiment thereof, more particularly provides a filament-wound plastic water heater tank in which a specially designed patch structure is utilized to reinforce an opening extending inwardly through the exterior filament winding and the tank wall.
Filament-wound pressure tanks, such as those incorporated in various types of water heaters, are typically constructed using an inner tank body which may be representatively of a blow-molded plastic construction. To reinforce the inner tank body it is exteriorly wound with a resin-impregnated filament, such as a fiberglass filament material, in a combination of helical and circumferential wraps such that the finished filament winding comprises a series of filament material layers. The applied filament winding on the exterior of the tank body is then cured to harden it to thereby substantially reinforce the tank body to permit it to handle internal operating pressure levels that It might not otherwise be able to withstand.
If the filament-wound tank requires the formation of a wall opening therein, such as a heating element sidewall opening in a water heater tank, it is necessary to cut the filament fibers to extend the opening into the tank interior. This hole-cutting operation substantially weakens the exterior reinforcing portion of the overall tank structure. To compensate for this weakening, one or more reinforcing patches are applied to the tank structure. These patches are designed to help tie the cut filament fibers to each other and to different layers of the filament winding.
Conventionally, these reinforcing patches are of a multi-layered triaxial design, with different fiber directions in the layers, and are customarily of a tight weave, sometimes being knitted to retain their shape and body. The multilayered tight-weave patches are applied to the tank within the various filament winding layers to reinforce the area and fibers that will be cut when the tank opening is subsequently formed. Each patch is soaked in the resin material to help bond it to the layers of the filament winding, with each patch being of a very tight weave and thick construction to help transfer the load in the different directions of the windings around the tank. The design goal of utilizing this type of patch structure is to cause the installed tight weave, multilayer patches to bond to the different layers of the filament windings with the resin soaked into the patch and the filament fibers.
In the formation of filament-wound tanks (as well as filament-based tanks of various non-wound varieties), this conventional patch reinforcing technique has several problems, limitations and disadvantages. For example, this type of patch structure is often prone to failure, thereby substantially weakening the strength of the tank, due to delamination of the various individual patch elements caused by stress on their outer layers by filament windings bonded thereto. This problem arises from the difficulty of adequately resin-bonding the various layers of each individual patch member to one another. Additionally, it is often difficult to assure that resin flows completely through each patch. Moreover, the conventional large thickness of these multilayer patches undesirably places additional stress on the contiguous filament layers by causing them to sharply bend around the patch edges.
As can be readily seen from the foregoing, a need exists for an improved hole-reinforcing patch technique in the production of filament-based pressure tanks such as the filament winding reinforced water storage tank portions of water heaters. It is to this need that the present invention is directed.